Generation and precise control of low and medium energy pulsed electron beams is required for many industrial, medical, and research applications, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), and horizontal/vertical accelerator-based beamlines (HAB/VAB), as well as relevant experimental analytical methods that use electron beams in SEM or TEM, or HAB/VAB as probes.
In research, pulsed electron beams with ultrashort pulse durations are used for investigating dynamic processes in a variety of materials. Frequently, the electron beams are combined with other primary excitation probes such as laser beams or other photon-based probes such as X-ray beams. An example would be the “pump-probe” class of experiments.
One approach for generating electron beam pulses of a specific length and charge (i.e. intensity) in a periodic sequence is to create electron pulses directly on the surface of an electron source (cathode) by exciting the electrons using either a laser or heat combined with an external electric field.
If a laser is used as the excitation method, the sequences of electron pulses are controlled by adjusting the wavelength, power, and/or temporal structure (pulse length and repetition frequency) of the laser photon pulses. For example, if a combination of femtosecond lasers and photocathode electron emitters is used, the electron pulse lengths are strictly determined by the pulse lengths of the fs-laser and the response time of the photocathode. Using this approach, it is possible to routinely obtain pulse lengths as short as 100 femtoseconds (“fs”) or less.
However, high repetition rates, defined herein as being repetition rates of at least 1 GHz or higher, are simply not available for laser-excited electron beams, because modern lasers are only capable of repetition rates on the order of 100 MHz or less (0.1 GHz or less).
In addition, it is often important in experimental systems to provide flexible and simple solutions for switching between continuous and pulsed beam modes. If the combination of a photocathode and an fs-laser is used for pulsed beam generation, then the required continuous beam must be generated using a separate thermionic or field emission source.
On the other hand, if heat combined with an external electric field is used as the excitation method, then the sequences of electron pulses are controlled by the electric field strengths and the temporal structure (pulse length and repetition frequency) of the electric field pulses.
Still another approach is to generate a continuous electron beam, and then to mechanically or electromagnetically block and unblock (i.e. “chop”) the beam with a desired periodicity, according to the desired electron pulse timing in the beam sequence. Approaches that use deflecting cavity technology for chopping electron beams of tens of kV in the GHz frequency range have been known since the 1970's. However, these approaches, which typically employ just one single-cell deflecting cavity, are generally limited to pulse lengths of 1 picosecond (“ps”) at best and repetition rates of 1 GHz or less, which cannot be changed or tuned. Furthermore, these approaches are only applicable for generating low energy electron beams having energies of less than 100 kilo-electron Volts (“keV”). Perhaps even more importantly, these approaches typically result in very extensive electron beam quality deterioration in both the transverse direction (beam diameter and divergence) and longitudinal direction (temporal coherence).
A combined ElectroMagnetic-Mechanical Pulser (“EMMP”) is disclosed in co-pending U.S. patent application Ser. No. 15/091,639, which has been published as US-2016-029337, and in an article published in Ultramicroscopy 161 (2016) 130-136, both of which are incorporated by reference herein in their entirety for all purposes. The EMMP disclosed in these references, referred to herein as a TDC-EMMP, is a device for generating electron beams that can be pulsed at a high duty cycle with pulsing rates greater than 1 GHz and with minimal transverse and longitudinal dispersion. The TCD-EMMP uses a Transverse Deflecting Cavity (“TDC”) to impose a spatial oscillation on a continuous input electron beam derived from any source. The spatially oscillating beam is then applied to an adjustable Chopping Collimating Aperture (“CCA”) so as to break the beam into a series of pulses, after which a dispersion suppressing section comprising a plurality of pillbox cavity resonators, cavity resonators, and/or magnetic quadrupoles is used to suppress temporal and spatial dispersion of the pulsed beam.
Referring to FIG. 1, a conceptual diagram is shown that illustrates the fundamental concepts underlying the TCD-EMMP approach. In the illustrated approach, an initially continuous, “DC” electron beam 100 is transversely modulated into a sinusoid 110 by a vacuum-filled TDC 102 which is operated at a resonant frequency that lies within a range between 1 GHz and 10 GHz. The amplitude of the sinusoid 110 grows as the modulated beam propagates, and then the beam 110 impinges upon a chopping, collimating aperture, or “CCA” 104, having an opening 106 that is adjustable between 10 μm and 200 μm. The CCA “chops” the beam into pulses 108 that emerge from the CCA at an ultrahigh repetition rate that is twice the TDC modulation rate, because the pulses 108 are produced by cutting the sinusoid 110 of the beam modulation on both the up-swing and the down-swing. The aperture opening 106 and the modulating field of the TDC tune the pulse lengths to between 100 fs and 10 ps, resulting in duty cycles of the EMMP device of less than or equal to 20%.
After the beam 100 has been chopped into pulses 108, if nothing further were done, both the longitudinal and lateral divergence of the stream of pulses 108 would increase. In other words, the pulses would get longer (temporal divergence in the propagation or “z” direction) and would spread out (spatial dispersion in the x and y directions). So as to avoid this, as shown in FIG. 1, additional components 112, 114 are included in a divergence suppressing section downstream of the CCA 110 that reverses and suppresses this divergence. In the embodiment of FIG. 1, the divergence suppressing section includes an additional, demodulating, TDC 114, which is identical in design to the modulating TDC 102, as well as a magnetic quadrupole 112. Additional details as to the features and underlying principles of the TDC-EMMP are presented in application Ser. No. 15/091,639, and in Ultramicroscopy 161 (2016) 130-136.
As is described in more detail in Ultramicroscopy 161 (2016) 130-136, the TDC-EMMP approach illustrated in FIG. 1 is highly effective in overcoming shortcomings that are inherent to earlier approaches, and fills an important gap in the space-time landscape of previously available devices for producing pulsed electron beams.
For example, UTEM (Ultrafast TEM) is also a stroboscopic pump-probe method, except that the pump and the probe signals are both laser-actuated. In UTEM, data are repeatedly collected over extended periods of time, and the heat that is imparted by the pump laser must be removed so that the process under study remains reversible. Therefore, even though lasers with higher repetition rates are available, UTEM systems typically operate at electron pulse rates of much less than 0.1 GHz, and sometimes even at approximately 0.1 MHz, depending on the experiment.
In contrast, a TDC-EMMP pulsed electron source can be used to enable real-time UTEM monitoring of processes that are driven electrically, magnetically, or both, and which can be cycled indefinitely at GHz frequencies, thus enabling truly “in operando” microscopy for observing processes such as switching in a semiconductor device. For example, when studying piezoelectric field effect transistors (FETs), a timed electron beam produced using a TDC-EMMP can be used to map charge distribution directly on top of an in operando tension/compression map of the piezo material, thus providing a direct visual correlation between the effectiveness of charge transmission and the strain in crystalline lattice medium. By applying the bilayer pseudospin FET concept, advancements already made in monochromatic valence electron energy-loss spectroscopy (VEELS) can then be used to map the full path and lifetime of excitonic transitions across the semiconducting bilayers.
In addition to imaging bottlenecks for charge propagation in logic-based materials, a stroboscopic TEM based on a TDC-EMMP can also be used to image spin and chemical discontinuities in memory architectures such as spin-torque, domain-wall, and phase-change RAM, and also for studying novel interconnect materials such as nanotubes.
This class of problems, characterized by electrical stimulus and lack of an extended cool-down time, is largely distinct from the class of problems typically studied in laser-based UTEM systems. Thus, a stroboscopic TEM based on a TDC-EMMP device, by incorporating a purely electronic approach to pump/probe electron microscopy, complements existing laser-based approaches by reaching much higher repetition rates for application in the study of processes that can be so driven.
The higher repetition rates provided by TDC-EMMP devices also have an immediate advantage in terms of the amount of time required to accumulate a measured signal, because of the potential for much higher duty cycles and thus much higher time-averaged probe currents.
As an alternative to the laser-photocathode combination method of producing electron pulses, blanking of a direct current (DC) electron beam using a TDC-EMMP can produce periodic electron pulse sequences with a flexible temporal structure that can be perfectly synchronized with the clock signal driving a high-frequency nanoscale device (be it a transistor, a spin-based memory, or another such device).
The basic principle of such stroboscopic TEM is presented in FIG. 2, and its key performance parameters are presented in Table 1.
TABLE 1initial beam energy200 keVintrinsic energy spread0.5 eVdc beam current at gun exit~100 nAoperation modestroboscopiclasernot requiredpulse length100 fs1 ps10 pssampling rate at specimen10-16 GHz5-16 GHz1-16 GHzduty cycle10−3 − 1.6 × 10−35 × 10−3 − 1.6 × 10−210−2 − 1.6 × 10−1(0.1-0.16%)(0.5-1.6%)(1-16%)number of electrons ~0.1~1~10per cyclerms emittance ≤0.4 nm × rad   ≤0.4 nm × rad≤0.4 nm × radinduced rms energy spread0.39 eV<0.1 eV<0.05 eVtotal energy spread 0.63 eV<0.51eV≈0.5 eV{square root over ((intrinsic2 + induced2))}total relative energy spread   3.15 × 10−6    <2.55 × 10−6  ≈2.5 × 10−6STR at 1 nm spatial<10−22 m · s<10−21 m · s<10−20m · sresolution
According to the approach illustrated in FIG. 2, an electron source 200 directs a continuous beam of electrons 202 to a TDC-EMMP 204, which produces a stream of electron pulses 206 that impinge on the device 208 that is under study. A small part of the RF signal 210 supplied to the TDC-EMMP 204 is diverted to the sample 208 through a phase-locked delay line 212, and is used to trigger a repetition of the process that is being studied. A rapid series of measurements 214 can then be obtained in real time.
Similar to UTEM, issues such as heating can be anticipated. Nevertheless, nearly in-operando examination of devices or device structures is feasible using a TDC-EMMP, since the TDC-EMMP is electromagnetically driven, so that long thermal cool-down times are typically not required. In particular, a stroboscopic microscope based on a TDC-EMMP can reveal the inner workings of advanced devices by concurrently enabling all of the high-resolution imaging, analytical, nanoscale diffraction (including strain measurements), and other capabilities (such as holographic imaging of electric fields and spectroscopic imaging of plasmonic fields combined with tomography) of a modern TEM on the timescales of the devices' normal operation.
Nevertheless, while the TDC-EMMP has many advantages, the use of a TDC to impose the spatial oscillation onto the beam imposes some inherent limitations. In particular, while the TDC has the advantage of being applicable to a continuous range of beam energies, i.e. electron velocities, it is only able to operate at a few distinct resonant frequencies. In other words, the spatial oscillation rate, and therefore the electron pulsing rate, is limited to only a few values that correspond to resonant modes of the TDC cavity. In reality TEM machines and many other applications to which the TDC-EMMP is applicable operate at a predetermined, fixed beam energy. Accordingly, many of the important applications to which the TDC-EMMP is applicable would benefit significantly from an EMMP having a highly broadband, continuously variable pulsing rate, rather than a continuously variable range of beam energies.
What is needed, therefore, is an EMMP that is able to produce a pulsed electron beam which is tunable over a wide range of pulsing frequencies (i.e. pulse repetition rates).